On-line calibration consists of calibration operations performed to maintain the ALMA observing system properly tuned to successfully execute the planned observations. The results are used in quasi-real time by the observing process, but also as input by hardware quality control, by the dynamical scheduler, and by the Science Pipeline. Among processing operations are the calibration of the atmospheric absorption, of phase radiometric correction, of pointing scans, of phase and amplitude reference measurements, . . . We describe the developments as planned and the options taken so far.